1. Field of the Invention
The present invention relates to reuse of a modulated input optical signal by flattening the modulated input optical signal in a reflective semiconductor optical amplifier (RSOA) or in a semiconductor optical amplifier (SOA) to convert into an upstream optical signal, and more particularly, to an operating system in which a signal having polarity opposite to that of the input optical signal is injected into the RSOA to improve the flatness of the input optical signal to reuse the input optical signal and an SOA structure that is suitable for the wavelength reuse method.
2. Description of the Related Art
In order to realize a wavelength-division-multiplexed passive optical network (WDM-PON) using a conventional RSOA, there has been suggested a method in which an optical signal that is modulated into downstream data by a central office (CO) is transmitted to the RSOA of an optical network terminal (ONT) and the RSOA is operated at a gain saturation region with respect to the power of the input optical signal to greatly reduce a difference between level ‘0’ and level ‘1’ of the input optical signal when the input optical signal is remodulated by upstream data. In such an optical signal reusing method, when the gain saturation of the RSOA occurs at lower optical power, an optical power budget in an optical link increases, and therefore, gain saturation power needs to be reduced to be as little as possible and an optical signal input to the RSOA needs to be amplified sufficiently for upstream transmission. Consequently, a gain of the RSOA needs to be large enough.
However, in actuality, the compressing ability of the RSOA in the gain saturation region is limited, and therefore, there is a limit in reducing an extinction ratio (ER) of the input optical sufficiently. In this case, when the residual ER is directly modulated by the upstream data again, the residual ER is reflected to the thickness of the level ‘1’. As the level ‘1’ is thicker, upstream transmission quality is getting worse. If the level ‘1’ is thicker than a certain thickness, the upstream transmission quality is getting worse rapidly. Accordingly, the ER of a downstream optical signal may need to be decreased to a minimum, which is just enough for transmission. At this time, due to the low ER of the downstream optical signal, power penalty may occur in downstream transmission. Especially, if the optical wavelengths of apparatuses that comprise a downstream link are even slightly misarranged, the ER of an optical signal input into a receiver is getting smaller than the ER of an optical signal output from a transmitter, so that sensitivity that the transmitting quality is getting worse rapidly is shown.